It is often desirable to use satellites to broadcast content, not least in areas where it is not commercially viable to use terrestrial transmitters.
Broadcasting content over a large area using a satellite requires a large amount of power. Current technology generally limits the amount of power continuously available on a satellite to less than about 20 kW. A satellite broadcast service, such as a digital television service, over an area the size of North America or sub-Saharan Africa, provided with current techniques is likely to result in a satellite requiring a continuous power of about 100 kW. A single satellite would therefore have insufficient power to provide a continuous service over a large continental area. For this and other reasons, it is desirable to provide a time divided broadcast signal. The satellite may direct a beam including the content at different geographical areas at different times. User devices on ground can receive the content in bursts and can display the content directly or store the content for later display. The user devices can be arranged to power up, in accordance with a schedule, and synchronise with the satellite in time to receive the content beam. However, it is sometimes desirable to cycle the transmissions rapidly and even to vary the content density between the different cells. The time it takes to power up and synchronise a user device puts constraints on the speed at which the transmissions can be cycled and the flexibility of varying the content density between the cells.
The invention was made in this context.